There are various mechanisms and methods for controlling moisture content of gaseous flow in air conditioning systems or other applications. One of the most popular and successful designs uses solid desiccant contact structures such as desiccant rotors or wheels. These desiccant wheels essentially comprise a honeycomb-type structure which are adapted to cycle through different gaseous flows. The desiccant contained on the structure adsorbs moisture from one gaseous flow (process side) and/or releases moisture contained in the desiccant to a gaseous flow (regeneration side), to regenerate the desiccant for future use.
In air conditioning systems, where both drying and cooling of the air are priorities, energy efficiency is extremely important. Normally, solid desiccant structures are open cycle systems in which the regeneration stream receives external heat energy input and is then discharged to the atmosphere after recovering moisture from the wheel, i.e. regenerating the desiccant. This results in a significant loss of heat in the regeneration stream/side, particularly if high temperature regeneration is used.
Closed loop systems in conjunction with desiccant contact structures have been proposed. U.S. Pat. No. 6,029,467 to Moratalla discloses a method and apparatus for conditioning air using a simple single desiccant wheel system. A first flow of air to be dehumidified passes through the desiccant wheel on the process side. A second flow of air, isolated from said first flow, passes through the regeneration side in a closed loop arrangement. A plurality of temperature changes are provided in the closed loop, via condensers or heaters for instance, to cool the air to below its dew point, remove the resultant moisture and then reheat the dried air so that it is in a suitable condition for regenerating the desiccant.
Similarly, U.S. patent. application Ser. No. 2009/0139254 describes a closed recirculation loop on at least the regeneration side of a desiccant wheel. Like U.S. Pat. No. 6,029,467, however, the regeneration stream needs to be cooled to below its dew point resulting in significant additional heat loss. This heat loss is exacerbated by an additional energy requirement to reheat the air back to the desired regeneration temperature for contact with the desiccant wheel, thereby negating any benefit of operating in a closed loop configuration.
Another variation of the closed loop regeneration site is disclosed in International Pat. Application No. PCT/NL2007/050495 (Publication No. WO 2008/044932). In this document, the process of regenerating the desiccant by means of superheated steam is disclosed. Again, however, this is for a single simple desiccant wheel system for drying food. There is no suggestion regarding whether the disclosed method and system would be suitable multiple desiccant structures or for air conditioning systems. The process develops a dried gaseous stream at an elevated temperature. Such an elevated temperature airstream supply is impractical for use in air conditioning systems in which the intent is to provide an air supply which is cooler and dryer than the ambient or conditioned space.
In large installations, of course, where the volume of gas/air flows and energy requirements are high, multiple desiccant wheels may be required. However, there is little research conducted in regard to multiple desiccant system in arrangements other than parallel, i.e. each desiccant contact structure/rotor operating independently. The applicant believes there are significant advantages available in arrangements and systems for multiple desiccant contact structures to provide reliable service particularly in air conditioning systems but also produce significant efficiencies in energy usage.
An example of a desiccant cycle with a pair of desiccant structures are disclosed in the U.S. Pat. No. 5,176,005. In that system, two desiccant contract structures are used with regeneration air for the second structure sourced from the secondary outlet of an indirect evaporative cooler to air in minimising the heat lost to ambient. However, after reheating and passage through the second desiccant structure this air stream, the heat energy contained within it, is subsequently lost to the ambient. This energy loss is common in open loop based desiccant based cycles, even when methods to recover heat are employed.
Use of condensation heat from vapour compression cycles to augment the regeneration heat of a desiccant system has been pursued in various inventions. For example, U.S. Pat. No. 5,564,281 describes a two coil condenser arrangement in the regeneration stream of a desiccant system for improved heat transfer. The recovery of the latent heat of condensation contained within humid air may of course be achieved by using a condenser coil supplied with an external source of coolant to cool the humid air below the dew point temperature. However, this requires energy input to keep the coolant below the dew-point temperature and so negates most or all of the benefit from recovering the heat of condensation.
Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field.